There is an easier way to go from flabby wimp to sinewy screen predator. Sometimes a superhero's journey begins with the needle prick of a syringe full of human growth hormone (HGH), testosterone, or steroids.

"In Hollywood, the drug of choice is the drug that makes you look good," says Strike Back's Winchester. "It's like the drug scene at a boarding school – it's all available." When actors ask about steroids, trainer Steve Zim tells them about the hair loss and zits, and "that usually ends the conversation in one second." Steroids also produce rounder, water-retaining muscles instead of the lean, mean bodies currently in vogue. Testosterone and HGH are far more common, particularly for older actors, since lower levels of testosterone can make it impossible to retain muscle mass. "Over 40? I encourage getting tested," says trainer Bobby Strom, "but there are some trainers who just go right to the testosterone, like they're putting you on a multivitamin."

Zim has seen the benefits of hormone therapy firsthand. "These people who look younger and fitter – a lot of them are using growth hormone and testosterone; the size comes from the testosterone, the virility and the youth come from the growth hormone."

On set, actors swap tricks of the fitness trade – and the phone numbers of trainers and doctors who will prescribe testosterone or HGH, no questions asked. There are dozens of hormone-replacement clinics in and around Hollywood, and their business is booming. But there are significant risks: Hormone therapy accelerates all cell growth, whether healthy or malignant, and can encourage existing cancers, especially prostate cancers, to metastasize at terrifying rates. Testosterone supplements can lower sperm counts. For many, the risk is worth it.

So who on a movie set would be most likely to take a risk on something unproven that could cause bodily harm? The stuntmen, of course. Several actors we spoke to say the stunt guys introduced them to performance-enhancing drugs. It makes some sense: If you're asked to body-double for Ryan Gosling without the benefit of his trainer and his personal chef, you'll be tempted to take a shortcut, too. And if you're jumping off buildings, battling ninjas, or swinging a battle-ax at ogres all day (or, worse, playing the ogre who gets bashed in 20 consecutive takes), you'll see an upside to HGH's accelerated recovery time.

Stuntmen often work for day rates, so every day they can't work is a day they don't get paid. "The stunt guys are partying hard, in their thirties or forties looking 20, 25," says one action star. "They're taking massive hits and bouncing back up again. I asked, 'What are you guys doing?' " According to the actor, a stuntman told him, "Steroids to get a build, insulin injections to get the cut, then HGH." Stuntmen talk about drugs as a calculated risk that's worth the advantage, so long as they get regular colonoscopies and screenings for prostate cancer. It's easy to see how an actor – especially one who relies on his brawn or his ability to throw a convincing punch – might seek that same edge.

That edge is what lured Manu Bennett, who played the fearsome gladiator Crixus on Spartacus. In 2007, at the age of 35, he got a lead role in The Smashing Machine, the story of ferocious Mark Kerr, an MMA fighter and drug addict. Bennett was to face off with Jean-Claude Van Damme. It was a dream role, his real break, so Bennett went all-in.

"I try to build myself as a physical representation of the character, and I knew Kerr had problems with steroids," says Bennett. "So it challenged me to, uh, fully embrace the role." (It's an echo of what Mickey Rourke said when asked about steroid use during his Oscar-nominated role as a veteran grappler in The Wrestler: "When I'm a wrestler, I behave like a wrestler." Or Tom Hardy's more caustic explanation of his Dark Knight Rises physique: "No, I took Smarties," he replied when a reporter asked if he'd juiced for the role. "What do you fucking think?") Bennett says he began doing two-a-day workouts with a former Mr. Australia and began taking injections. He put on 44 pounds in three months.

When he arrived on the set in early 2008, he boasts, "I could have challenged the look of people like Stallone and Schwarzenegger." But then the star, 47-year-old Jean-Claude Van Damme, never arrived and the movie was never shot. According to Bennett, Van Damme didn't want to be shown up. "He'd seen a picture of me, and I was absolutely pumped," Bennett says, "and he was not in the best shape of his life at that time." Bennett is still angry. "This fuck-up just thinks of himself. He looks at this photo of me and feels egotistically challenged." (Van Damme's representatives did not respond to requests for comment.)

In the end, Bennett's big break had broken him: "I ended up going back with no money and had to work on a building site as a day laborer for eight months, swinging that pick, jackhammering."

And since he could no longer afford the hormone supplements, his estrogen levels surged. "It was horrible," he says, noting that he would sob uncontrollably on the work site. "Everything went kind of soft. I was on my period for two months, crashing on the estrogen."

Eight months later, he got a call from his agent. Ironically, the producers of the new TV series Spartacus had seen a photo of Bennett at his drug-enabled peak and cast him as the show's villain, Crixus. Bennett left the construction site and hit the gym, but says he never went back on the juice. He gained back much of his size, but not all of it.

Now Bennett is 44 – old for a badass nemesis – and won't rule out hormone therapy in the future. "For the roles I take on, I've got to be an actor and a professional athlete or fighter to somehow match their myth," he says. "I've got to set a new summit and figure out how I'm going to get up there."